js: Use SOURCE_CONTINUE/SOURCE_REMOVE constants in source functions

With support for boolean constants in g-i, we can finally use the
more readable constants instead of true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/ui/panel.js

@@ -628,6 +628,7 @@ const ActivitiesButton = new Lang.Class({
 
         Mainloop.source_remove(this._xdndTimeOut);
         this._xdndTimeOut = 0;
+        return GLib.SOURCE_REMOVE;
     }
 });